1. Field of the Invention
This invention relates to a nozzle plate of an ink jet apparatus. In particular, this invention relates to a method for forming higher quality nozzle apertures on the nozzle plate and a nozzle plate structure permitting the forming of higher quality nozzle apertures.
2. Description of Related Art
U.S. Pat. No. 5,208,604 discloses a conventional fabrication method for forming nozzles in a nozzle plate of an ink jet apparatus. A nozzle plate, forming part of an ink jet head for ejecting ink, has a plurality of nozzles through which the ink is ejected. The nozzle plate is formed from a polymeric material such as polyimide polyethersulfone. The nozzles are formed by exposing the nozzle plate to an excimer laser beam using a mask having transparent portions corresponding to areas of the nozzle plate in which the nozzles are to be formed. When exposed to the excimer laser beam, the exposed portions of nozzle plate absorb the excimer laser beam, thus separating a molecular bond in the polyimide polyethersulfone. The polyimide polyethersulfone molecules and the atoms freed from their bonds decompose and spread to make nozzle apertures, as described in "Ultraviolet Laser Ablation of Organic Polymers", Chemical Reviews, Vol. 89, No. 6, pages 1303-1316, 1989.
In this fabrication method, an inner diameter of the nozzle on an excimer laser entrance side of the nozzle plate is larger than an inner diameter of the nozzle on an excimer laser exit side of the nozzle plate. Preferably, the inner diameter of the nozzle on an ink jet side should be smaller than the inner diameter of the nozzle on the inside of the ink jet head. For this reason, the surface of the nozzle plate on the inside of the head is exposed to the excimer laser beam.
However, this conventional nozzle fabrication method, as disclosed in U.S. Pat. No. 5,208,604, encounters a problem that laser workability is considerably deteriorated when a fluorine-based or silicon-based repellent film is applied to the ink jet side of the nozzle plate to improving the ink ejection property of the nozzle. The repellent film repels water and oil, and is used when the nozzle plate is formed from, for example, polyimide polyethersulfone.
The fluorine-based or silicon-based repellent film does not absorb the ultra-violet wavelength radiation generated by excimer lasers. For example, a krypton-fluorine (KrF) laser emits at a wavelength of 248 nm, while a xenon-krypton (XeKr) laser emits at 308 nm. Hence, the repellent film is not processed by the excimer laser. Rather, the repellant film is processed only by the heat and kinetic energy resulting from the decomposition and spread of molecules and atoms of the plate. As previously mentioned, in terms of the shape of the nozzle, the inner diameter of the nozzle on the ink ejection side should be smaller than the inner diameter of the nozzles on the inside of the ink jet head. For this reason, a surface of the nozzle plate, which is opposite to the surface on which the repellent film is formed, is exposed to the excimer laser. Since the heat and kinetic energy resulting from the decomposition and spread of molecules and atoms of the nozzle plate and transmitted to the repellent film is small, the repellent film is not suitably processed. Thus, the geometrical and dimensional accuracy of the nozzle deteriorates by two or three times relative to the required accuracy. When the heat and kinetic energy resulting from the decomposition and spread of molecules and atoms of the nozzle plate is small and the distribution of the energy varies, the shape of apertures formed on the ink ejection side is degraded. As a result, the direction in which the ink drops are propelled as they are ejected varies, resulting in poor print quality.